A syndiotactic polypropylene has been known for a long period of time. Such a polypropylene is prepared by polymerization at a low temperature in the presence of a conventional catalyst comprising a vanadium compound, an ether and an organic aluminum compound, but the thus prepared syndiotactic polypropylene is poor in syndiotacticity and it scarcely has syndiotactic characteristics.
A polypropylene having good tacticity, i.e., a syndiotactic pentad fraction of more than 0.7 in accordance with .sup.13 C-NMR has been discovered for the first time by J. A. Ewen et al. which can be obtained by polymerizing propylene in the presence of a polymerization catalyst comprising compounds of transition metals (Hf and Zr) having an asymmetric ligand and methyl aluminoxane (J. Am. Chem. Soc., 110, 6255-6256, 1988).
In the J. A. Ewen et al. method described above, the activity per transition metal is high, and the obtained syndiotactic polypropylene is excellent in syndiotacticity and physical properties inclusive of impact resistance. However, this kind of syndiotactic polypropylene has the problem that stiffness is poor. In addition, when propylene is copolymerized with another olefin by the use of the above-mentioned method, a copolymer can be obtained which is composed of propylene and the other olefin and which has a substantially syndiotactic structure. However, the thus obtained copolymer is difficult to crystallize, and when molded, there is the same problem that the above-mentioned syndiotactic polypropylene has.